1. Field of the Invention
The present invention relates to a photodetector device which converts an incident light to an electrical signal according to a control voltage supplied from the external, and a photodetection circuit including a photodetector device.
2. Description of the Prior Art
A photodetector having a metal-semiconductor-metal structure can be used as a photoconductor having variable sensitivity. For example, Y. Nitta, J. Ohta, S. Tai and K. Kyuma, "Variable-sensitivity photodetector that uses a metal-semiconductor-metal structure for optical neural networks" (Optics Letters, 16(8), 611-613 (1991)) discloses a photodetector having two metallic electrodes formed on a gallium arsenide (GaAs) substrate. Schottky contacts are formed at interfaces between them. The output signal of the photodetector is connected to an external circuit of a trans-impedance amplifier. The sensitivity of the photodetector can be controlled by changing a bias voltage applied to the electrodes. The direction of the photocurrent is reversed by changing the polarity of the bias voltage. An effective negative sensitivity can be obtained by changing the direction of photocurrent, or the photocurrent can be suppressed low. If the bias voltage is zero, no photocurrent is taken out due to the Schottky barrier. Dark current is small due to the existence of the Schottky barrier.
A two-dimensional photodetector array comprising a plurality of the photodetectors having a metal-semiconductor-metal structure is used for devices such as an optical neuro-device and an artificial retina device. For example, refer to K. Kyuma, E. Lange and Y. Nitta, Optoelectronics-Devices and Technologies 8(1), 35-52 (1993) and E. Lange, E. Funatsu, K. Hara and K. Kyuma, Proceedings of 1993 International Joint Conference on Neural Networks, pp.801-804 (1993).
However, the sensitivity of the photodetector formed in a GaAs substrate is affected largely by the state at the interfaces or by the crystalline state of the GaAs substrate, so that it has bad reproducibility. Further, a GaAs substrate is expensive than a silicon substrate, and integration techniques on a GaAs substrate are not so mature at present.
On the other hand, a photodetector having a metal-semiconductor-metal structure formed in a silicon substrate has disadvantages of low Schottky barriers, and this makes it difficult to set the zero point and to suppress dark current. Therefore, it is difficult to adopt a silicon substrate for the metal-semiconductor-metal structure.
Further, negative sensitivity is needed for various image processing such as Fourier transformation and contour detection, and a power source of negative voltage is needed to be included in an external circuit in order to obtain the negative sensitivity. However, it is a problem that the power source of negative voltage makes the external circuit complex. It is also a problem that the photodetector itself does not have mechanisms for charge storage and amplification in contrast to an image device such as a charge coupled device, and the sensitivity of the photodetector is much smaller than that of the prior art image devices.